On Wed, Sep 22, 2021 at 10:33 AM Peter Xu [off-list ref] wrote:

Hello, Axel,

On Wed, Sep 22, 2021 at 10:04:03AM -0700, Axel Rasmussen wrote:

quoted

Thanks for discussing the design Peter. I have some ideas which might
make for a nicer v2; I'll massage the code a bit and see what I can
come up with.

Sure thing.  Note again that as I don't have a strong opinion on that, feel
free to keep it.  However if you provide v2, I'll read.

[off-topic below]

Another thing I probably have forgot but need your confirmation is, when you
worked on uffd minor mode, did you explicitly disable thp, or is it allowed?

I gave a more detailed answer in the other thread, but: currently it
is allowed, but this was a bug / oversight on my part. :) THP collapse
can break the guarantees minor fault registration is trying to
provide.

I think your approach of checking the VMA flags *in
retract_page_tables specifically* is correct, and a similar thing
should be done for minor registered VMAs too.

When I'm reworking the uffd-wp series, I noticed that commit e1e267c7928f
("khugepaged: skip collapse if uffd-wp detected", 2020-04-07) was actually
awkward and not efficient, as we can simply lookup the vma flags for detecting
uffd-wp enablement.  I'm preparing a patch for it to do it by checking vmas
(and that patch will also pave the way for file-backed).

Then I noticed we need similar thing for minor mode?

I think the answer is yes, but I didn't see any code that explicitly handled
thp for minor mode, do you remember?

To be explicit, what if in mcontinue_atomic_pte() we get a shmem_getpage() call
with a thp returned?  Will minor mode break?

Ah so this I am not quite as sure about.

The issue I was describing in the other thread was more about THP
collapse racing with UFFDIO_CONTINUE. E.g., collapsing after
registration has happened, but before faults have been resolved.

But there's another scenario: what if the collapse happened well
before registration happened? I *think* the existing code deals with
THPs correctly in that case, but then again I don't think our selftest
really covers this case, and it's not something I've tested in
production either (to work around the other bug, we currently
MADV_NOHUGEPAGE the area until after VM demand paging completes, and
the UFFD registration is removed), so I am not super confident this is
the case.

I plan to post the khugepaged patch soon and I plan to cover minor mode too
there, but I'm not sure whether that's enough, as the thp can be there from the
1st day I think, but I could have missed something.

--
Peter Xu